The invention relates to a plasma display panel comprising, as shown in FIGS. 1A, 1B, a first plate 11 and a second plate 12 leaving between them a space filled with a discharge gas and compartmentalized into a number of discharge cells 18 arranged in rows and columns, which also includes an array of insulating barrier ribs comprising barrier ribs 15 each separating two adjacent columns of cells, the first plate including at least two arrays of coplanar electrodes Y, Y′ called sustain electrodes, which are oriented in general directions that are parallel to one another and perpendicular to said barrier ribs, having a constant width perpendicular to these general directions, and are arranged in such a way that each discharge cell is traversed by an electrode of each array.
Since the barrier ribs 15 each separate two adjacent columns of cells, these barrier ribs are called column barrier ribs, as opposed to row barrier ribs described later.
Each discharge cell is therefore traversed by a pair of sustain electrodes and each pair of sustain electrodes therefore supplies a row of discharge cells; all the adjacent cells of any one row are separated by a column barrier rib made of insulating material; in this way, in the general direction of the coplanar electrodes, the widths of the various cells in any one row are limited by these column barrier ribs. These barrier ribs generally serve as spacers between the plates of the panel.
The coplanar electrodes are covered with a dielectric layer 13 which is itself coated with a protective/secondary-electron-emissive layer 14, generally based on magnesia.
The second plate includes a third array of electrodes X called address electrodes, each placed between two column barrier ribs. Thus, each address electrode therefore supplies a column of discharge cells. These address electrodes may also be covered with a dielectric layer 17.
The array of barrier ribs in certain panels of the prior art also include barrier ribs 16 called row barrier ribs each separating two adjacent rows of cells, in such a way that each cell of the panel is therefore bounded, over its entire perimeter, by barrier ribs as shown in FIGS. 1A, 1B.
The operation of driving the plasma panels conventionally includes address periods intended to activate those cells that have to be turned on, followed by sustain periods during which series of sustain voltage pulses are applied between the sustain electrodes Y, Y′ supplying a row of cells, and the gap G separating these electrodes. The amplitude of these sustain pulses must be sufficient to cause discharges in those cells in the row that have been actuated beforehand but insufficient to cause discharges in the cells of this row that have not been activated beforehand.
The addressing of the discharge cells generally takes place between a column electrode and one of the row electrodes, which also serves for sustaining.
The discharge cells and the space between the plates are filled with a low-pressure gas suitable for obtaining discharges that emit ultraviolet radiation.
The walls of each cell are generally provided with a layer of a phosphor capable of emitting visible radiation, especially in the red, green or blue, when it is excited by the ultraviolet radiation of the discharges. These layers are generally deposited on the second plate and on the side walls of the barrier ribs.
In the case of panels emitting three primary colors, namely red, green and blue, these adjacent discharge cells have phosphors of different colors so that discharges emitting indirectly in the red, the green and the blue are obtained.
It is in general the first plate, the one bearing the coplanar electrodes, which serves as the front plate turned toward the person observing the images that the panel is capable of displaying. To prevent the electrodes of the front plate absorbing too great a portion of the visible radiation coming from the cells, the coplanar electrodes are preferably made of a material that is both conductive and transparent, such as tin oxide or mixed indium tin oxide (ITO); as these transparent electrodes are not in general conductive enough, the arrays of transparent electrodes are generally “duplicated” with opaque metal conductors, called “bus conductors” since they distribute the electrical discharge current to the transparent electrodes. Conventionally, the linear electrical conductivity of the bus is greater than that of the initiating conductor. The bus is made of a highly conductive metallic material, such as silver, and consequently it is opaque to light.
During a sustain period, when an electrical voltage pulse of sufficient amplitude is applied between two coplanar electrodes Y, Y′ of any one pair, in a cell supplied via these electrodes and activated beforehand during an address period, a discharge is initiated in the gap G near the initiation edge 191 of one of these electrodes, over a front that extends between the column barrier ribs 15 that define, widthwise, this cell at this point. As shown in FIG. 1A, the discharge is initiated in this cell in an initiation region Za of the portion of this electrode that corresponds to this cell. It is preferable for the surface potential properties of the dielectric layer 13 coating this electrode to be sufficiently uniform to allow initiation of the discharge at low voltage. After initiation, the discharge spreads out perpendicular to the general direction of the coplanar electrodes as far as the end-of-discharge edge 192 of the electrode, on the opposite side from the initiation edge. The phase during which the discharge spreads out, called the expansion phase, allows the formation of a discharge region with a low electric field, this being very effective for exciting the gas and producing ultraviolet photons. The expansion phase therefore improves the luminous efficiency of the discharges. During the expansion phase, when the discharge expands up to the end-of-discharge edge of the electrode, the discharge occupies almost all of the gas space bounded by the two column barrier ribs 15 that define the width of the cell.
During a sustain period, immediately before an electrical voltage pulse has been applied between two coplanar electrodes Y, Y′ of any one pair traversing a cell, the dielectric layer region that covers these electrodes is generally covered with residual charges called “memory charges”, coming in particular from the previous discharge in that cell. Immediately at the start of application of an electrical voltage pulse and before any new discharge, the discharge gas region lying between these two electrodes is then subjected to the sum of the voltage applied between these electrodes and of the voltage resulting from the memory charges coming from the previous sustain pulse.
FIG. 3 shows, at the start of a sustain voltage pulse of 100 V amplitude applied to the electrodes, which follows from other identical AC pulses that have left memory charges, the distribution of the equipotential voltage lines in a cross section on A1-A1′ of the discharge expansion region, between the middle of a column barrier rib 15 and the middle of the cell, this range corresponding to half the distance between the centers of two adjacent column barrier ribs, that is to say the half-width of a discharge cell. The equipotential lines, shown as continuous lines, correspond to positive values of the potential while the equipotential lines shown as broken lines correspond to negative values of the potential. The potential difference between two adjacent equipotential curves is constant and suitable for obtaining twenty “positive” equipotential curves shown as continuous lines. During the initiating 100 V voltage pulse, it is assumed here that the electrode in question, Y, acts as cathode and that the negative memory charges stored in this cell on the surface of the dielectric layer 13 come from the discharge generated by the previous sustain voltage pulse of the same series, but of opposite sign. In this figure, the equipotential curve V corresponds to the first negative equipotential (shown in broken lines, as opposed to the continuous lines of the positive equipotentials) and indicates the presence of a negative charge deposited at this point on the surface of the column barrier rib 15. The distribution of this equipotential depthwise in the column barrier rib indicates that, after initiation caused by the pulse in question, the discharge will spread out over the side walls of the barrier ribs, and therefore beyond the surface of the dielectric layer 13 and the protection layer 14 covering the electrode Y. During sustain periods in which the panel emits light, the barrier ribs will therefore be in substantial contact with the discharges. This phenomenon results in bigger losses of the charged species on the barrier ribs and to accelerated deterioration of the phosphor material covering these barrier ribs with, as a consequence, a reduction in the luminous efficiency and a reduction in the lifetime of the panel.
The prior art, illustrated for example by document EP 0 782 167 (PIONEER), proposes a solution to this problem that is shown in FIG. 2. FIG. 2 shows a schematic top view of the structure of a cell of a coplanar plasma display panel that differs from the structure shown previously in FIGS. 1A and 1B in that the coplanar electrodes no longer extend over the entire width of the cells. Each electrode Y includes a continuous conductive bus Yb at the end-of-discharge edge 192 that traverses all the cells of any one row and, in each cell, an electrode element Yp in the form of a tongue centered on this cell, having a width smaller than this cell and extending from the bus as far as the initiation edge 191. The electrode elements Yp of each cell are sized in such a way that their lateral edges are positioned at a non-zero distance D from the surface of the closest column barrier ribs 15 that define this cell.
Such a structure applied to the coplanar electrodes Y, Y′ makes it possible to reduce the potential on the side walls of the column barrier ribs and on the surface portions of the protective layer that are close to these barriers along the lateral edges of the electrode elements Yp, as illustrated in FIG. 4, which shows the distribution of the electrical equipotential curves in the cell shown in FIG. 2, in a cross section on A2-A2′ in the mid-width of the cell, and of the same assumptions and conventions as for FIG. 3 described above. This FIG. 4 indicates that the first negative equipotential curve, shown in broken lines, meets the V-shaped column barrier rib at the top of this rib, at the interface with the protective layer and the dielectric layer 13.
It follows from these dielectric properties, illustrated by the equipotential curves, that there is better confinement of the sustain discharges away from the column barrier ribs at the start of expansion in the panels described in document EP 0 782 167 or, with reference to FIG. 2, relative to the panels described previously with reference to FIGS. 1A and 1B. Thus, the luminous efficiency and the lifetime are improved.
However, at the end of expansion of the discharges, that is to say at the buses Yb of the coplanar electrodes, the same problem as previously is encountered since the electrodes extend at this point over the entire width of the cells. The potential along the barrier rib surface and the surface of the protective layer remains high near the electrode portions Yb corresponding to the buses. The improvement in luminous efficiency and in lifetime therefore remains limited.
Furthermore, such a structure having electrode elements is more difficult to produce than that of FIGS. 1A and 1B and requires an expensive operation of horizontal alignment of the plates 11 and 12 so that the electrode elements specific to each cell are perfectly centered on each cell and equidistant from two adjacent column barrier ribs.